Foam control in separation of hydrocarbons



Aug. 2, 1966 INVENTOR. RE. LUKEN BY @4 QM?! A TTORNE'KS' United States Patent O 3,264,365 FOAM CONTROL IN SEPARATION OF HYDROCARBONS Robert E. Luken, Borger, Tex., assignor to Phillips Petroleum Company, a corporation of Delaware Filed June 7, 1965, Ser. No. 467,809 12 Claims. (Cl. 260-683.3)

This application is .a continuation-in-part of my copending application Serial No. 160,068 led December 18, 1961.

This invention relates to the separation of hydrocarbons by means of a solvent. In one aspect this invention relates to control of foam in an extractive distillation of hydrocarbons wherein a solvent is utilized. In a more specific aspect this invention relates to the detection of incipient flooding conditions in an extractive distillation operation and .to the prevention of such ooding.

In a commercial process for the manufacture of butadiene, including the steps of butane dehydrogenation to butenes, butene recovery or purification, butene dehydrogenation to butadiene, .and butadiene recovery or purification, the use of furfural `as `a selective solvent in the extractive distillation of butenes and butadiene has made the process practical and economical. The use of furfural permits separations of C4 lhydrocarbons which were heretofore considered either impractical or impossible on a commercial scale. Such separations are necessary for the production of high purity butadiene.

One of the major problems encountered in the extractive distillation of C4 hydrocarbons for the recovery of butenes and butadiene and for the separation of butadiene from butenes has been that of flooding or stacking in .the extractive distillation column as la result of furfural foaming. It has heretofore been discovered that certain polymeric organo-silicon compounds are useful for reducing the foaming of furfural in Van extractive distillation operation and a method for reducing furfural foaming has been disclosed and claimed in U. S. Patent 2,750,435 issued lune l2, 1956, to John Fetchin. The use of organosilicon polymers such as liquid polydimethyl siloxane has been found Ito be extremely beneficial in providing a measure of control of furfural foaming and hence column stacking in the extractive distillation of C4 hydrocarbons. Since these siloxanes are expensive materials and are not recoverable in the operation, it is desirable to use the minimum amount of material which will provide satisfactory operation of the extractive distillation columns. Experience has indicated that `about 1 or 2 parts per million of a polymeric organo-silicon compound such as polydimethyl siloxane will provide satisfactory operation when the extractive distillation column is operating at normal conditions. Although the practice of maintaining 1 or 2 parts per million of the siloxane in the furfural solvent employed in the extractive distillation process provides, under normal conditions, satisfactory operation, there still occur periods of unbalance in the extractive distillation operation which result in column stacking and necessitate reduction of feed to the columnas well as reduction of heat input to the column until normal operation can be resumed. The cause or causes of such periods of unbalance have not been determined. The ope-ration of an extractive distillation column is a delicate operation which requires a proper balance of a great number of variables including temperature, pressure, feed rates and withdrawal rates. Any change in ope-rating conditions which tends to upset this delicate balance can result in stacking in the column.

It is an object of this invention to provide a method for preventing the occurrence of flooding or stacking in an extractive distillation operation. It is also an object of this invention to provide a means for detecting incipient 3,264,365 Patented August 2, 1966 stacking conditions and for effecting prevention of such stacking. A still further object of this invention is to provide a means for sensing conditions leading to stacking in a column and to introduce into the column a material to prevent such stacking. Other objects and advantages will be apparent to one skilled in the-art upon study of this disclosure including the detailed description of the invention and the appended drawing wherein:

The sole `figure is a schematic flow sheet illustrating the operation of the invention.

Broadly, the invention contemplates the prevention of stacking in an extractive distillation column by dumping into the column, preferably at a lower portion thereof, an amount of a polydimethyl siloxane equal to about the amount which is normally added over an 8-hour period when the differential pressure from the bottom of the column to the top of the column exceeds a value indicating incipient stacking. Stacking or flooding of the column is prece-ded by an increase in the differential pressure across the column resulting from the increased quantity of liquid on the trays of the column. Attempts to alleviate this condi-tion by increasing the quantity of siloxane in the furfural added to the column have been unsuccessful and in each instance it has been necessary to reduce the feed of hydrocarbon and solvent to the column and to reduce the input of heat to the column until ythe excess liquid has been vaporized and removed. The column can then be brought back to normal operation gradually. I have discovered that stacking of the column can be avoided by the introduction of a large slug of siloxane into the lower portion of the column when the pressure drop yacross the column indicates that the column is in a state of unbalance which, experience has shown, results in stacking of the column.

Since the invention herein described is directed to .the concept of preventing stacking in a distillation column during the extractive distillation of a hydrocarbon, and

' since the extractive distillation technique can be employed for separating `a variety of hydrocarbons, it is apparent that the invention can be applied to any extraction wherein a less saturated hydrocarbon is separated from a more saturated hydrocarbon by means of a solvent. For example, the invention can be practiced in connection with the separation of hydrocarbons wherein the individual molecules contain from three to seven or more carbon atoms each. Thus, the invention can be used to prevent stacking in an extractive distillation zone which is used to separate a less saturated C3 hydrocarbon from a more saturated C3 hyd-rocarbon, such as propylene from propane, or a less saturated C4 hydrocarbon from a more saturated C4 hydrocarbon, such as butene from butane. The invention can also be used to prevent stacking in an extactive distillation zone wherein a solvent is used to separate a less saturated C5 hydrocarbon from a more saturated C5 hydrocarbon, such as pentene from pentane or pentadiene from pentene. Further uses for the invention include processes of separating heptene from heptane, normal hexane from cis-2 hexene, and cyclohexane from isoheptenes.

The invention may be more readily understood by reference to the drawing wherein an ext-ractive distillation column comprising two sections is represented by the lower section 10 and the upper section 11, each of which contains 50 conventional bubble cap trays. Hydrocarbon feed is introduced into the lower section 10 at about the 41st 4tray via conduit 12. Heat is supplied to the kettle of the column by a heater indicated at 13 which can be a steam coil or other conventional heating element. The overhead vapors from the lower section 10 are passed via conduit 14 to the lower portion of the upper section 11. Liquid from the bottom of the upper section 11 is passed via conduit 15, pump 16 an-d conduit 17 to the A ance of the extractive distillation column.

top of the upper section 10. The kettle product from the;-

ylower section 10, comprising rich furfural solvent, is

passed via conduit 18 containing motor valve 19.*to the stripper 21 for regeneration ofthe solvent and lean solvent is removed from the bottom of stripper 21 via conduit 22 containing motor valve 23` and passed through pump 24 and cooler 25 and introduced into the upper portion of upper section 11. Valve 19 is actuated by liquid level controller 26 and valve 23 isactuated by liquid level controller 27. A proportioning pump 28 adds siloxane from a reservoir 29 to conduit 22.1 Reservoir .31 contains a quantity of polydimethyl siloxane equivalent to that added over a period of about 8 hours by proportioning pump 28. Siloxane is introduced into conduit 15 as a single v dump via conduit 32V and motor valve 33. An equalizing line 34 connects the top of reservoir 31 with conduit 15* downstream from pump 16. Motor valve 33 is actuated by dilerential pressure .controller 35 which senses the pressure `at the top -of section 11 and the bottom of sec.-

Vtion rof the extractive distillation column and opens V valve 33 when the diierential of these two pressure valves exceeds a predetermined value. The diierential pressure.

controller closes valve 33 when the differential pressure -returns to normal and Vreservoir 31 is then ready to be recharged with siloxane for the next occasion of unbal- Ithe reservoir 31 with the siloxane is accomplished by forc. ing a measured quantity of siloxane into reservoir 31 via conduit 36. The siloxane displaces the heavier furfural downwardly through conduit 37 containing check yalve 38 Yand into conduit 15.

The overhead vapors from the extractive distillation step are removed via conduit 41, cooled andV condensed in cooler 42, and passed to accumulator 43 from whence the condensate is removed via conduit 44 by pump 45, a

measured portion of the .condensate -being returned .t-o

section 11 as reflux via conduit 46 containing motor valve 47 which is operated by rate of flow controller 48. The remainder of -the condensate is removed `as product via conduit 49, the quantity being governed by liquid level controller 51 actuating motor valve 52 in conduit 49.V The overhead vapors from stripper 21 are removed via conduit 53, cooled and condensed in cooler 54 and passedy to accumulator 55; A portion of the condensate yis re- Y moved from accumulator 55 via conduit `56and passed vi-a pump 57 and conduit 58 to the top of stripper 21` as.

reux as controlled by rate .of ow controller 59 which Charging of f controls valve 61 .in conduit 58. Excess condensate as determined by liquid level controller 62 is removed from 150 accumulator via conduit 63 containing motor valve 64V which is actu-ated by liquid level controller 62.

A Differential pressure controller is composed of the con-` trollererecorder 35 which can be, for example, a Foxboro Model 58-P2 controller, Model 56 Recorder; and the differential pressure transmitter 35a which can be a Foxboro Type 13A d/p cell, differential pressure transmitter. Such instruments are well known and are commercially available. The controller is set at the value at which action v is to be taken. For example, the controller can be set at 10 p.s.i. differential so that valve 33 is opened wide when the differential pressure is above about 10.1 Vp.s.i. andi will be completely closed at 9.9 p.s.i.

The siloxanes which have been found effective in the processof the invention are the vorgano-silicon polymers represented by the empirical formula Corning Corporationas DC 200. Fluids and DC Antifoam A.

The continuous additionof siloxanes to reduce foaming of furfural-in the extractive distillation of C4 hydrocarbons is described in the above-referred to Patent 2,750,435.

The drawing illustrates a typical operation whereinthe process and apparatus of the claims has been successful inr preventing or decreasing the severity `of ooding in the extractive distillation step in the 'separation of n-butane and butene-Z. The extractive distillation column or absorber ;is.`l31/2 feet in diameter and is divided into two SO-tray. sections. .The hydrocarbon feed isintroduced .onto

theV 41st tray and the furfural Vsolventis introducedonto the 96th tray. The .ratio of solvent to hydrocarbon is about 61/2 to l on a volume. basis. The column pressure is about p.s.i. at the top and about 65 p.s.i. at thebottom. The kettle' temperature is about 310 F.and the top temperature is about 120'F.

Duringthe run. now beingconsidered.siloxane (DC 200 Fluid).V was. added continuously to the furfural feed to the absorber at -a rate .of about l p.p.m. (volume) per pass of furfural through the` absorber. The siloxane was added as a solution of siloxane 4in deoiler bottoms (C5 and heavier hydrocarbons including.Y those containing .102

carbon atoms or more kper molecule obtained as kettle product in the: distillation of C4 hydrocarbons in the process of dehydrogenation of C4 hydrocarbons to form butadiene) as described in Patent 2,750,435.

The pressure drop'r across the absorber was relatively,

steady at 4 to y5 p.s.i. for about 20 hours of the .days run v under consider-ation and thenbegan to increase rapidly to about 11 p.s.i. so that they reservoir 31,!containing two gallonsof a 50-50 mixture of siloxane in deoiler bottoms, was Vdumped into the lower portion of the absorber. This amount of siloxane is equivalent-to about 5 p.p.m. (volume) based on the per pass of furfural through the absorber.; The pressure `drop across the absorber returned.

to normal within about= 30 minutes and continued to operate at normal for the rest ofthe day. The next day the pressure .drop was normalffor the first 14 hours of operation, then began-5to uctuate between 2 `and 6 p.s.i..

After about4 hours of rough operation, as indicated by v the uctuating pressure drop, the, pressure drop began to increase rapidly indicating that flooding or stacking was beginningrto occur land at 10 p.s.i. diierential pressure the.V reservoir containing two gallons of a 50450 mixturer of siloxane and deoiler .bottoms was dumped into theV lower portionof the absorber.'y The pressure diierential from'bottom to top of the absorber dropped to normal iin about one hour. and the .absorber continuedto operate by allowing the use of the ,minimumY amount necessary` Prevention of the occurrence.

during normal operation.

of flooded conditionsfin the absorber also prevents unbalof the stripperwith the .overhead vapors, necessitatingv wherein n is an integer resulting in a compound having a viscosity of at least 40 centistokes, and preferably at least 100 centistokes at 25 C. and maybe classed as polydimethyl siloxanes. Polydimethyl siloxanes suitable for use in the process of the invention are marketed by Dow ance in operations downstream from the absorber; for example, a flood in the absorber. means that there is an `excess of liquid in theY absorber so that the .feed to the.

furfural stripper is thereby reduced. Areduction in-the feed to the strippercan cause furfural vto be passed out reprocessing of Athe .stripper overhead vapors to recover the furfural.;

than about 5 ppm., basedon a per. pass of furfural through theA absorber, canbe added to the.'y absorber to stop ailoodingcondtion which lhasstarted. It isalso possible that in some situations a smaller amount of siloxane will be sucient;y however, since the cause of It is to be understood .that a greater amount of siloxanei the -iooding is not known in each instance, I prefer to add an amount at least equivalent to about `5 p.p.m. (volume) based on the per pass of furfural through the absorber.

Although the invention has been described in connection with the extractive distillation step in the separation of n-butane from butene-Z, it is obviously within the scope of this invention to employ the inventive concept in connection with the separation of various other hydrocarbons including propylene from propane, pentene from pentane, and the like. In these other applications of the invention, the polydimethyl siloxane can be employed in the same concentrations and in the same manner as describe-d in connection with the' separation of n-butane from butene-Z. Thus, the siloxane can be added in substantially pure form or it can be added in solution in a liquid hydrocarbon containing from about 5 to about carbon -atoms per molecule as described in U.S. Patent 2,750,435.

Variations and modifications are possible within the scope of the disclosure of the invention without departing from the spirit and scope of the invention.

That which is claimed is:

1. In a process for separating butene-Z from n-butane in an extractive distillation zone in the presence of a solvent comprising furfural to which is added at least about l ppm., per pass of furfural through said distillation zone, of liquid polydimethyl siloxane of the formula wherein n is an integer resulting in a compound having a viscosity of at least 40 centistokes at 25 C., the improvement comprising adding to the lower portion of said distillation zone a single increment of said siloxane equivalent to labout 5 to l0 p.p.m. by volume based on the per pass of furfural through said distillation zone when the pressure drop across the distillation zone exceeds a value indicating incipient ooding of the distillation zone.

2. The process of claim 1 wherein the `siloxane is added in substantially pure form.

3. The process of claim 1 wherein the siloxane is added as a solution in a liquid hydrocarbon containing from about 5 to 10 carbon atoms per molecule.

4. In a process for the manufacture of olens from normal butane which comprises, catalytically dehydrogenating normal butane to said olens, subjecting the C4 hydrocarbon content of the reaction eluent to fractional distillation so as to remove the butene-l content thereof as `an overhead product and produce a bottoms product containing principally the n-butane and butene-2 content thereof, subjecting the resulting mixture of n-butane and butene-2 to extractive distillation with a solvent comprising essentially `a mixture of furfural, l to 60 Weight percent of C5C10 hydrocarbons formed in said process and comprising principally butadiene dimer, and liquid polydimethyl siloxane of the formula wherein n is an integer resulting in a compound having a viscosity of at least 40 centistokes at 25 C. in a concentration of about 1 to 2 p.p.m. by Volume per pass of furfural so as to selectively -dissolve butene-2, and stripping the dissolved butene-2 from the rich solvent, the improvement comprising continuously determining the pressure drop across the distillation, -and adding to the lower part of the distillation a single increment of said polydimethyl siloxane equivalent to about 5 to l0 ppm. by volume based on the per pass of `furfural through said distillation when the pressure drop across the `distillation indicates a state of unbalance which will result in looding in the distillation.

5. In a process for separating a less saturated C4 hydrocarbon from a more saturated C4 hydrocarbon wherein a mixture of said hydrocarbons is subjected to an extractive distillation step in the presence of furfural solvent and at least about 1 p.p.m., based on the furfural passing through the `distillation step, of liquid polydimethyl siloxane which is added to sai-d furfural substantially continuously, said polydimethyl siloxane being of the formula the distillation step exceeds a Value indicating incipient flooding.

6. In a system for the extractiva distillation of C4 hydrocarbons to separate a less saturated C4 hydrocarbon from a more` saturated hydrocarbon comprising an extractive distillation column, a stripping column to strip absorbed hydrocarbon from rich solvent, means to pass rich solvent from said distillation column to said stripping column, conduit means containing a pump to return lean solvent from said stripping column to said distillation column conduit means containing a pump to introduce C4 hydrocarbons to said distillation column, means to recover less saturated C4 hydrocarbons from said stripping column, and means to recover more saturated C4 hydrocarbons from said distillation column, apparatus comprising dumping means to introduce a single ldump substantially instantaneously into said distillation column a measured quantity of a liquid polydimethyl siloxane of the formula v wherein n is an integer resulting in a compound having a viscosity of at least 40 centistokes at 25 C.; pressure sensing means to sense the difference in pressure between the bottom and the top of said distillation column; pressure control means operatively connected to said sensing means and to said dumping means to dump said measured amount of said siloxane into said distillation column when the diierence in pressure between the bottom and the top of said distillation column exceeds a value indicating incipient ooding of said distillation column.

7. The apparatus of claim 6 wherein said dumping means comprises a closed vessel of lsutlcient volume to contain the measured quantity of said siloxane; a dumping conduit connecting the bottom portion of said vessel to a conduit to introduce lluid to said distillation column at a point upstream from said pump; a motor valve in -said dumping conduit operatively connected to said pressure control means; a relief conduit connecting the upper portion of said vessel to the conduit to introduce fluid to said distillation column at a point downstream from `said pump; a displacement conduit -connecting the lower portion of said vessel to said conduit to introduce iluid to said distillation column; a check valve in said displacement conduit to prevent flow of uid into said vessel; and means to force a measured amount of said siloxane into the top of said vessel against the pressure in said conduit to introduce uid to said distillation column.

8. In a process including an extractive distillation step in a distillation zone in the utilization of a solvent comprising furfural in which foaming is prevented by the addition of liquid polydimethyl siloxane of the formula te re es om-sro-sli- -o-si-on3 CH3 ons n om 10. The process of claim 8 Awherein the siloxane is.

added as a solution in a liquidhydrocarbon containing from about to 10 carbon atoms per molecule.

11. In a process for separating a less saturated hydro-I carbon from a more saturated hydrocarbon wherein a mixture of said hydrocarbons is subjected to an extractive distillation step in the presence of furfural solvent in which foaming is prevented by the addition of liquid polydimethyl siloxane of the formula CH3 CH3 n CH3 wherein rt is an integer resulting in a compound having a viscosity of at least 40 centistokes at 25 C., the improvement comprising adding to the lower portion of said dis-` tillation zone a single increment of said siloxane equivalent to about 5 to l0 p.p.m. by volume based on the per pass of furfural through said distillation zone whenthe pressure drop across the distillation zone exceeds a value indicating incipient liooding of the distillation zone.

12. In a system forextractive distillation comprising an t extractive distillation column, a stripping column to strip rich solvent, means-to pass rich solvent from said distilla` tion column to said stripping column, means to return lean solvent from said stripping column to said distillation column, means to introduce hydrocarbons to said distillation column, means to remove less,V saturated hydrocarbons from'saidstripping column, and means to remove more saturated hydrocarbons from said distillation column, fapfy paratus comprising dumping means to introduce a single dump `substantially instantaneously into said distillation column a measured `quantity ofA a liquidpolydimethyl siloxane of the formula wherein n is an integer resulting in a compound having a viscosity of at least 401centistokes at 25 C.; pressure sensing means to sense thedilerencein pressure between the bottom and the top of said distillation column; pressure control meanstoperatively connected to said sensing means and to said dumping means to dump said measured amount of said siloxane into said distillation column when the difference, in pressure between the bottom andthe topV of saiddistillation column exceeds a value indicating in-` cipient ooding of said distillation column.

References` Cited bythe Examiner UNITED STATES'PATENTS 2,232,840 2/'1941 Claley 202-160 2,750,435V 6/1956 Fetcbin 260-680k 2,816,858 f 12/.1957 Walker 202-160 3,000,795rk 9/ A1961 G'Oeldn'er 202l60 3,027,307 3/1962 DELBERT E. (Misra-Primm Examiner.

S. P. JONES, Assistant Examiner.

Stoier etal 202--32 

4. IN A PROCESS FOR THE MANUFACTURE OF OLEFINS FROM NORMAL BUTANE WHICH COMPRISES, CATALYTICALLY DEHYDROGENATING NORMAL BUTANE TO SAID OLEFIN, SUBJECTING THE C4 HYDROCARBON CONTENT OF THE REACTION EFFUENT TO FRACTIONAL DISTILLATION SO AS TO REMOVE THE BUTENE-1 CONTENT THEREOF AS AN OVERHEAD PRODUCT AND PRODUCE A BOTTOMS PRODUCT CONTAINING PRINCIPALLY THE N-BUTANE AND BUTENE-2 CONTENT THEREOF, SUBJECTING THE RESULTING MIXTURE OF N-BUTANE AND BUTENE-2 TO EXTRACTIVE DISTILLATION WITH A SOLVENT COMPRISING ESSENTIALLY A MIXTURE OF FURFUURAL, 1 TO 60 WEIGHT PERCENT OF C5-C10 HYDROCARBONS FORMED IN SAID PROCESS AND COMPRISING PRINCIPALLY BUTADIENE DIMER, AND LIQUID POLYDIMETHYL SILOXANE OF THE FORMULA (CH3-)3-SI-(O-SI(-CH3)2)N-O-SI(-CH3)3 WHEREIN N IS AN INTEGER RESULTING IN A COMPOUND HAVING A VISCOSITY OF AT LEAST 40 CENTISTOKES AT 25*C. IN A CONCENTRATION OF ABOUT 1 TO 2 P.P.M. BY VOLUME PER PASS OF FURFURAL SO AS TO SELECTIVELY DISSOLVE BUTENE-2, AND STRIPPING THE DISSOLVED BUTENE-2 FROM THE RICH SOLVENT, THE IMPROVEMENT COMPRISING CONTINUOUSLY DETERMINING THE PRESSURE DROP ACROSS THE DISTILLATION, AND ADDING TO THE LOWER PART OF THE DISTILLATION A SINGLE INCREMENT OF SAID POLYDIMETHYL SILOXANE EQUIVALENT TO ABOUT 5 TO 10 P.P.M. BY VOLUME BASED ON THE PER PASS OF FURFURAL THROUGH SAID DISTILLATION WHEN THE PRESSURE DROP ACROSS THE DISTILLATION INDICATES A STATE OF UNBALANCE WHICH WILL RESULT IN FLOODING IN THE DISTILLATION. 